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JPA stands for Java Persistence API, which basically is a specification that describes a way to persist data into a persistent storage, usually a database. We can think of it as something similar to ORM tools like Hibernate, except that it is an official part of the Java EE specification (and it’s also supported on Java SE).

There are many reasons to learn an ORM tool like JPA. I will not go into the details of this because there are already many posts on the web which perfectly answer this question, like this one, or this one. However, we should also keep in mind that this is not a single magic bullet which will solve our every problem.

When I first started out with JPA, I had real difficulties to set it up because most of the articles on the web are written for Java EE environment only, whereas I was trying to use it in a Java SE environment. I hope that this article will be helpful for those who wish to do the same in the future.

In this example we will use Maven to set up our required dependencies. Since JPA is only a specification, we will also need an implementation. There are many good implementations of JPA available freely (like EclipseLink, Hibernate etc.). For this article I have chosen to use Hibernate. As for the database, I will use MySQL.

Let us first create a simple maven project. I have created mine using the quick start archetype from the command line. If you do not know how to do that, you can follow this tutorial.

OK, so let us get the dependencies for the JPA next. Include the following lines in your pom.xml –

The first dependency specifies the standard JPA interface, and the second one specifies the implementation. Including JPA dependencies this way is desirable because it gives us the freedom to switch vendor-specific implementation in the future without much problem (see details here). However we will not be able to use the latest version of the API this way because the API version 1.0.2 is the last version that is released as an independent JAR. At the time of writing this article, the latest version of the JPA specification is 2.1 which is not available independently (there are lots of requests for it though). If we want to use that one now then our only options are to choose from either a vendor-specific JAR or use an application server which provides the API along with its implementation. I have decided to use the API specification provided by Hibernate. In that case including only the following dependency will suffice –

Now it’s time to configure our database. I will use the following schema in all of my future JPA examples which I found from this excellent online book –

Database Schema

Create an equivalent database following the above schema in your local MySQL installation.

Our next step is to create the persistence.xml file which will contain our database specific information for JPA to use. By default JPA expects this file to be in the class path under the META-INF folder. For our maven project, I have created this file under project_root/src/main/resources/META-INF folder –

The above file requires some explanation if you are an absolute begineer in JPA. In my next article I will try to explain it as much as possible, but for running this example you will only need to change the first three property values to match your environment (namely the database name, username and password). Also keep a note of the value of the name attribute of the persistence-unit element. This value will be used to instantiate our EntityManagerFactory instance later in the code.

Ok, let us now create an entity to test our configuration. Create a class called Address with the following contents –

This class has been properly mapped to the address table and its instances are fully ready to be persisted in the database. Now let us create a helper class called PersistenceManager with the following contents –

If you check your database, you will see that a new record has been inserted in your address table.

This article explains how to set up JPA without using any other frameworks like Spring. However it is a very good idea to use Spring to set up JPA because in that case we do not need to worry about managing entity managers, transactions etc. Beside setting up JPA, spring is also very good for many other purposes too.

That’s it for today. In the next article I will try to explain the persistence.xml file and the corresponding configuration values as much as possible. Stay tuned!

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This is a continuation of an introductory discussion on Generics, previous parts of which can be found here.

In the last article we were discussing about recursive bounds on type parameters. We saw how recursive bound helped us to reuse the vehicle comparison logic. At the end of that article, I suggested that a possible type mixing may occur when we are not careful enough. Today we will see an example of this.

The mixing can occur if someone mistakenly creates a subclass of Vehicle in the following way –

/**
* Definition of Vehicle
*/
public abstract class Vehicle<E extends Vehicle<E>> implements Comparable<E> {
// other methods and properties
public int compareTo(E vehicle) {
// method implementation
}
}
/**
* Definition of Bus
*/
public class Bus extends Vehicle<Bus> {}
/**
* BiCycle, new subtype of Vehicle
*/
public class BiCycle extends Vehicle<Bus> {}
/**
* Now this class’s compareTo method will take a Bus type
* as its argument. As a result, you will not be able to compare
* a BiCycle with another Bicycle, but with a Bus.
*/
cycle.compareTo(anotherCycle); // This will generate a compile time error
cycle.compareTo(bus); // but you will be able to do this without any error

This type of mix up does not occur with Enums because JVM takes care of subclassing and creating instances for enum types, but if we use this style in our code then we have to be careful.

Let’s talk about another interesting application of recursive bounds. Consider the following class –

The reason for this is that even though Child inherits all the setters from its parent, the return type of all those setter methods are of type Parent, not Child. So the first setter will return reference of type Parent, calling setAttrib6 on which will result in compilation error, because it does not have any such method.

We can resolve this problem by introducing a generic type parameter on Parent and defining a recursive bound on it. All of its children will pass themselves as type argument when they extend from it, ensuring that the setter methods will return references of its type –

Notice that we have to explicitly cast this to type T because compiler does not know whether or not this conversion is possible, even though it is because T by definition is bounded by Parent<T>. Also since we are casting an object reference to T, an unchecked warning will be issued by the compiler. To suppress this we used @SuppressWarnings(“unchecked”) above the setters.

When writing method setters this way, we should be careful as to not to use recursive bounds for any other purposes, like to access children’s states from parent, because that will expose parent to the internal details of its subclasses and will eventually break the encapsulation.

With this post I finish the basic introduction to Generics. There are so many things that I did not discuss in this series, because I believe they are beyond the introductory level.

Until next time.

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I have been going through some very big changes in my life over the past year. I have changed my job, and moved with my whole family to Tokyo. I have been so busy with these stuff that I could not find enough time to write blogs.

I am still trying to adjust to my new life here at Tokyo. So far things have been a little rocky, but I am slowly settling down.

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This is a continuation of an introductory discussion on generics, previous parts of which can be found here.

In this post I will focus on type parameter bounds and their usage.

Bounded Type Parameters

When a generic type is compiled, all occurrences of a type parameter are removed by the compiler and replaced by a concrete type. The compiler also generates appropriate casting needed for type safety by itself during this procedure. This concrete type is typically Object, but compiler can use other types as well. This process is called Type Erasure and will be explained in a future post. For the time being, all we need to understand is that the type information of generic types are lost once they are compiled. For this reason, if we want to access a method/property using a type parameter, we’ll typically be able to access those ones that are defined in the class Object (I am oversimplifying here as we’ll be able to access other methods/properties as well if we use a bound, which we will discuss here in this post). For example, take a look at the following code snippet –

Although the message seems cryptic, the reason behind this is the one that I’ve stated above – when this type is compiled the type parameter E here will be replaced by Object by the compiler, and it doesn’t have intValue method.

So the problem occurs here because the compiler is using Object to replace the type parameters. If I could somehow tell the compiler to use other types during this erasure which has an intValue (Number, for example) method, then this error would have been resolved.

This is exactly what parameter bounds do. By using a type as a bound on a type parameter, I can instruct compiler to use that type during the erasure in place of Object, and then I can easily access the methods/properties defined in that type.

The general syntax for specifying a type parameter bound is as follows –

This also tells the compiler that when a type argument is passed during an instance creation of this generic type, it will be a subtype of MyBoundType, so it can safely let us access the methods that are defined in that type using the type parameter E. If any other type is passed, then the compiler will issue a compile-time error. The extends keyword specify the bound relation between E and MyBoundType. We will use the same keyword even if E is bounded by an interface type, that is, if MyBoundType is an interface. Here extends means both classical extends and implements.

So, if we use Number as our parameter bound for our last example, the error message will be gone because now the compiler will use Number to erase type parameter E, and it has an intValue method defined in it –

This told the compiler that the type arguments that will be passed here will all implement the Comparable interface, so they will have a compareTo method. As a result, compiler allowed us to do this inside the sort method –

This example also shows that we can pass another generic type as a type parameter bound. In fact we can use all Classes, Interfaces and Enums and even another type parameter as a bound. Only primitive and array types are not allowed as a bound.

Multiple and Recursive Bounds

We can define multiple bounds on a single parameter. In this case we use the & operator to separate them from one another –

This tells the compiler that the type argument that is passed will be a subclass of MyBoundClass and implement MyBoundInterface. So, we will be able to access all the methods/properties defined in those types.

The Java Language Specification requires us to list the class bound first, otherwise the compiler will throw an error. For example, the following will throw an error –

/**
* Will throw an error because we are not
* listing the class bound first.
*/
public class MyGenericClass
<E extends MyBoundInterface
& MyBoundClass>

We can also declare recursive bounds, so that a bound can depend on itself too. Consider again our stack example from first part of the series. We declared it like this –

public class InsertionSort<E extends Comparable<E>>

Here, the bound is recursive because E itself depends upon E (the one that is supplied to Comparator). If we pass Integer as a type argument when creating an instance of InsertionSort, then the type argument to Comparable will be Integer too.

Java Enum Declaration

Let us now consider an example from the Java API itself. We all know that enumerations in Java are objects of some class, and that class extends the Enum class. The declaration of that class looks like this –

Beginners in Java Generics find the first line very confusing. Before explaining the reasoning behind this weird declaration, let us explore an example.

Suppose that we are going to build a software system which will have various types of vehicles (a vehicle simulation system, perhaps?). The vehicles will have a name and a size. We also want to compare vehicles with each other based on their size. An approach for building the vehicle classes might look like this –

The problem of the above implementation is pretty obvious – even though the compare method implementations are almost the same among all the subtypes of Vehicle, we still have to maintain different copies of this method. This creates a maintenance problem as now changing the comparison logic forces us to change the code in many places.

To remedy this, we can remove the comparison from the subtypes and move it up in the Vehicle. To do this, we will rewrite those classes as follows –

This is certainly a problem, since comparing a bus with a car should not be done using the same logic that is used to compare a car with a car.

How can we solve this? How can we reuse the comparison logic among all the subtypes, while at the same time raising error flags at compile time whenever someone tries to compare two incompatible types?

In the last example the problem occurred because the compareTo method has a parameter which is of type Vehicle. As a result we were able to pass any subtypes of Vehicle to it, like the way we passed a bus to compare with a car. Let’s try to change the type of this parameter so that now this kind of mixing generates an error. If we want to allow the comparison of a car only with a car, then the argument to the compareTo method must be of type Car. If we change it to Car, we will also need to change the type argument that we are passing to Comparable in the Vehicle class declaration –

But then this will not allow us to compare any other types. We will not be able to compare a bus with another bus. To allow this, we will need to change the parameter to be of type Bus. If we declare a new subtype named Cycle, we will also need this method to support this type too! So we can see that the parameter type of this compare method should vary if we need to enforce compatible comparison.

From the above discussion it’s clear that we need to parameterize the parameter type of the compareTo method, and in turn, parameterize the Vehicle class itself. If we do this, we will then be able to pass Car, Bus, and Cycle etc. as its type argument, which in turn will be used as the parameter type of the compare method. In general, after we declare Vehicle as a generic type, all of its subtypes will pass themselves as a type argument while extending from it, so that the parameter type of this compareTo method matches their type –

public abstract class Vehicle<E> implements Comparable<E> {
// other methods and properties
public int compareTo(E vehicle) {
// method implementation
}
}
/**
* Now this class’s compareTo version will take a Car type
* as its argument.
*/
public class Car extends Vehicle<Car> {}
/**
* Now this class’s compareTo version will take a Bus type
* as its argument.
*/
public class Bus extends Vehicle<Bus> {}
/**
* Doing something like this will now generate a
* compile-time error.
*/
car.compareTo(bus);

This approach solves our last problem that we were facing, but introduces a new one. After converting Vehicle to a generic type and using the type parameter as the parameter type of the compare method, it looks like this –

Since we didn’t put any bound on the type parameter, and Object class doesn’t have a getLength method, compiler will generate an error. We get to call this method on a variable of type E only if it’s bounded by Vehicle itself, because then compiler will know that variables of this type will have this method. So our compare method will work only if E is bounded by Vehicle itself! After this modification, the classes look like below –

Even with the above example, a certain kind of type mixing is possible. Rather than discussing it here, I am going to leave it to you to figure it out. If you can’t, check out the next post of this series!

I guess now you know why the Enum class is declared in that way. This kind of recursive bound allows us to write methods in a supertype which will take its subtypes as its arguments, or returns them as return value. I encourage you to check out the source code of the Enum class to find out these methods.

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This is a continuation of an introductory discussion on generics, previous parts of which can be found here.

Before jumping into subtyping in generics, I thought it will be good if I talk a little about generic methods and automatic type inference.

Generic Methods

Just like generic class, we can define generic methods in Java. A generic method is a method which defines its own type parameters. In this case, the type parameter’s scope is limited to the method where it’s declared. To declare a generic method, we just list the type parameters before its return type, like this –

If this was an instance method, then we would have called it like this –

myInstance. <String>myFirstGenericMethod("Hello World");

Notice how we have passed the type argument while calling the method. Generally, the rule is as follows –

class/instance_var. <type_argument_list>methodName(argument list…..);

Our above method call will also compile without specifying the type parameters explicitly. This happens because of type inference – the java compiler’s ability to automatically determine the type argument during generic method call/generic instance creation. We will talk more about this in a few moments.

Let’s write another useful generic method which will add all elements of an array into a list –

Type Inference

Type inference is Java compiler’s ability to look at generic method invocation and instance creation to determine the type arguments that make the code work. This algorithm is capable of determining the type of the arguments and the type that the result is being assigned or returned, provided that there is enough information for the inference algorithm to do so.

The inference algorithm determines the types of the arguments and, if available, the type that the result is being assigned, or returned. Finally, the inference algorithm tries to find the most specific type that works with all of the arguments.

There are two situations where type inference algorithm comes into play –

When an object of a generic type is being created

When a generic method is being invoked

Type Inference during generic instance creation

Generally, type inference for instance creation works like this – first, the compiler tries to deduce the type arguments from the constructor arguments in the object creation expression (the one with the new). If it fails to deduce the type arguments for any of the type parameters from there, it then uses information from the context in which the creation expression appears.

In one of my previous posts, I have demonstrated a short-cut way to create generic instances without specifying the type argument –

Here, we see that the instance that is being created is being assigned to a variable, which is of type MyPrettySimpleGenericClass<Integer>. This line will be valid only if the generic instance being created also has the Integer as its type argument (no, any subtype/supertype of Integer will also not do, we’ll see why in a future post). When type inference algorithm tries to deduce the type argument that will be applicable for this instance creation expression, it first takes a look at the constructor arguments. Since the above code doesn’t have any, it then takes a look at the left hand side of the assignment operator and sees that only Integer can make the statement valid. Thus it automatically passes Integer as the type argument to the generic instance being created.

If constructor arguments are available and if the inference algorithm can determine the applicable type arguments from them, then it completely ignores the assignment context. Let’s consider an example –

List<Number> list = new ArrayList<>(Arrays.asList(0L, 0L));

We are passing long values to the asList method, which takes an arbitrary number of arguments as its input, and returns an object of type List containing those elements. As a result, the inference algorithm determines its return type to be List<Long>, and returns an instance of this type. This returned value is then again used by the inference algorithm to determine the type argument of the ArrayList being created, which is finally set to Long as well. Since List<Long> and List<Number> are totally incompatible type (again, more on this in a future post), the compiler generates an error (although in Java 8 it compiles fine).

What happens if there are no constructor arguments, or assignment of the generic instance being created? Let’s consider an example of that too –

Iterator<String> it = new ArrayList<>().iterator();

In the above example, the object creation expression occurs inside a chain of method calls. As the type inference for object creation expression is only allowed for an assignment context (this is only true for JDK 7 as JDK 8 has improved the inference algorithm a bit by including the method invocation context), the compiler is unable to determine the correct type. As a result, a compile-time error is issued by the compiler.

In the above example, the type inference cannot determine the applicable type arguments from the context, and will throw a compilation error.

Type Inference during generic method invocation

Type inference algorithm for generic methods works in the same way as the instance creation. Here, type inference first observes the method arguments to determine the type parameter, and then observes the context in which it is being used.

If we recall our myFirstGenericMethod method, we can see that the type inference is capable of determining the appropriate type argument by observing the string argument. Same thing is true for our addAll utility method.

What happens if the method doesn’t have any arguments which correspond to its type parameter? In this case the compiler tries to determine the type from the method invocation context. Let’s write a method which will create lists for us if we specify the initial capacity –

Our utility method doesn’t have any arguments which correspond to the type parameter. Hence inference algorithm cannot deduce anything by observing the arguments in the method invocation. It then tries to observe the calling context and sees that the result returned by the method is being assigned to a reference of type List<Integer>, and determines the type argument to the method to be Integer.

What if the above method is not being called from an assignment context? The compiler cannot determine the type argument! If we call our previous printAll method like this –

printAll(createList(10));

then the compiler will throw an error (JDK 8 can determine the type argument). We have to specify the type argument explicitly in this case, or introduce a variable to hold the value returned by the first method, and then pass it as argument to the latter.

We can declare generic methods in a generic class declaration too. What happens if the type parameter of the generic method matches with the type parameter of the generic class? I leave this to the reader to experiment!

Lastly, the Java Specification says that we can only specify type arguments when we call generic methods using dot notation. As a result, calling our createList method like this will result in a compile-time error (obviously from inside the class where it’s defined) –

<Integer>createList(5); // Error, won’t compile

In this case, if the generic method is a static one, we use class name to invoke the method. If it’s an instance method, we use an instance reference (or this / super) to call it.

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This is a continuation of my introductory tutorial on Generics. Previous posts in this series can be found here.

Why should we use Generics

I intended to write about this after I’ve demonstrated some generics examples so that their usefulness become apparent to the readers. So let’s see what the official tutorial of Java has to say about the usefulness of generics, or, why should we use them –

Stronger type checks at compile time

Elimination of casts

Enabling programmers to implement generic algorithms

The first two are very closely related. We have already seen how generics allow us to write generic algorithms. It’s time to focus on the first two.

If you are new to Collections, don’t worry, they are not that hard to learn. For the time being, you can simply consider myList to be something like an array, or a collection, where we can add elements using the add method, and later traverse them using a standard iterator.

This is how we used collections in the pre-generic era. To be used as a collection for any types, the add method expects an Object as its argument and casts/boxes any other types to it. As you can see from the above code, one can easily put integers into a collection of strings if they are not careful enough, just like we did in line number 9. In this case the program will compile without any trouble, but we will get a ClassCastException at runtime at line number 13, because we cannot cast an integer into a string.

Some people might consider my example far-fetched, saying that the scenario described above is less likely to occur in real world. I would like to tell them that the above scenario is a very simplified one. For example, you might call a library method in your program which returns a collection of strings, and you didn’t know it in advance and put integers in that list, resulting in runtime exception. If this is also not convincing, consider the Date class in Java which is defined in two different packages – java.util and java.sql, and mixing them together in the same collection will likely result in a runtime exception. It was programmer’s responsibility to make sure that a collection didn’t contain elements from various types, and programmers did make those mistakes.

When we use generics, the above problem doesn’t occur. Now it’s the compiler’s responsibility to make sure that a collection doesn’t get polluted by containing elements from more than one type. When we try to put an integer into a collection of strings, compiler will generate an error at compile time, rather than at runtime –

Using generics in this way ensures that our list will contain only elements of a certain type, and we will not run into any runtime exception. From the java best practice guidelines, we know that catching and fixing errors at compile time is much easier than catching them at runtime (more on this in a future article).

Point number two – elimination of casts

If you check out the second example, you’ll see that the cast at line number 13 is no longer required. This is because when we used generics, we have explicitly told the compiler that this collection will only contain a collection of strings. We will add only strings to it, and we will retrieve only string values from it. If you could check the method signature of the next method now (just like I did using Eclipse by hovering the cursor over this method), you’ll see that it too returns a string value where in the first example it returned an object of type Object. Eliminating cast this way also helps us to reduce the possibility of encountering any runtime exception (although implicitly this is a consequence of the first point).

Point number three – enabling programmers to implement generic algorithms

I have already explained this point in the previous two articles. If like to visit them again, here is the first one and here is the second one :-).

With the above three usefulness, I’d like to mention another one which I have described below.

Point number four – enabling programmers to write clear and concise code

Let’s write a simple program – put five numbers into a list and add them together. Here’s how we did it before generics were introduced –

You can probably read this code without much explanation. It’s clear and much shorter from it’s non-generic counterpart. Here we have made use of couple of other Java language features – the for-each loop and autoboxing/unboxing. Indeed generics work well with these features.

I hope you are now convinced about generics :-).

That’s it for today. Hope to write another post real soon. Stay tuned!

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This is a continuation of an introductory Generics tutorial, previous part of which can be found here.

Generics Instantiation Syntax

In the previous post we have learned how to declare a generic class. We have also learned its formal definition. Now it’s time to learn how to create instances of the generic class, formally.

To create an instance of a generic type, we do generic type invocation, which replaces the type parameter with a concrete type. We have seen multiple examples of this in the previous article. Remember our MyPrettySimpleGenericClass from the previous post? If you don’t, here is the definition –

In the above code, we have performed a generic type invocation by replacing the type parameter with a concrete type Integer, which makes this object to work with all the integer values. We call Integer a type argument in this case. An invocation of a generic type in this way creates what we call a parameterized type. Our parameterized type in this case is MyPrettySimpleGenericClass<Integer>.

You can pass any non-primitive type as a type argument. You can even pass another parameterized type too, because they are also non-primitive. Consider the following example –

In the above example, the type parameter is another parameterized type, which was obtained by parameterizing the List<E> generic interface (it’s part of the standard Java API, specifically the Collection API and I intend to write something about them too!).

You can see that this generic method invocation thingy looks somewhat similar to method invocation, except that during a method invocation you pass values as arguments (yes, Java is pass-by-value, and if you are wondering about it, please have patience as I intend to write a post about it in future), and here you pass types as arguments.

Now, some of you might be wondering about the constructor syntax of a generic type, after seeing the <Integer> part right before the parentheses in the above code. Do we also have to put type parameters in the constructor of a generic type?

Short answer – No. You declare a constructor of a generic type like any other constructor. You don’t need to put a type parameter right before the constructor parentheses (in fact you’ll get a compilation error if you try something like that) – putting it right after the class name is enough, unless your constructor itself is a Generic Method (I promise I won’t run away until I write a post about it) which introduces a new type parameter!

In JDK 7 and later, you can omit the type argument during the invocation of the constructor of the generic type, provided that the compiler can determine, or infer(yes, Type Inference will be coming up in a future post), the type argument from the context. Taking advantage of this feature, we can modify our previous object instantiation in the following way –

We generally use arrays to store stack elements. This approach won’t work here, because we cannot create an array of generic types. Instead, we will be using the Linked List implementation.

We have created an inner class to represent a linked list element. This is a perfect example of when an inner class should be used. We wanted to make a type which will store an element of type E, and will also hold a reference to the next element. If we make it a top-level class, we won’t be able to access the type parameter E from it. So, we needed a type which will have access to internal information of another type. As a result, we use an inner class (yes, more on inner classes will come next, in the near future, perhaps within the next ten years…..hmmmmmm……..).

Notice that we have declared our inner class as private. If we don’t declare it as such, then it may also be accessed from outside Stack, thus leaking our implementation detail in the outside world.

Notice the declaration of push, pop and peek method. The first one throws an UnsupportedOperationException if an element is pushed while the stack is full, and the last two throws a NoSuchElementException when these operations are done in an empty stack. Some might think of using other exception classes, or building their own with more customization. As for this example, it’s a simple one, so I will be using these two.

See how we are using our stack for different types? Aren’t generics awesome 😀 ?

Play with it as long as you want. Tweak the example in as many ways as you like. Try creating an Integer stack and pushing a string into it. Do it the other way round. See what errors/exceptions you get. Also, try to implement other data structures, like Queue, with generics. See how awesome they become!